The present invention relates generally to sputtering, and more particularly, to high power impulse magnetron sputtering (HIPIMS).
Sputtering is a physical process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions. The process of sputtering is commonly used for thin-film deposition. The energetic ions for the sputtering process are supplied by a plasma that is induced in the sputtering equipment. In practice, a variety of techniques are used to modify the plasma properties, especially ion density, to achieve the optimum sputtering conditions. Some of the techniques that are used to modify the plasma properties include the usage of RF (radio frequency) alternating current, an AC power source, a DC power source, a superposition of DC and AC power sources, a pulsed DC power source such as a bipolar or unipolar power source, the utilization of magnetic fields, and the application of a bias voltage to the target.
Sputtering sources are usually magnetrons that utilize magnetic fields to trap electrons in a closed plasma loop close to the surface of a target. The electrons follow helical paths in a loop around the magnetic field lines. The electrons undergo more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur. The sputter gas is inert, typically argon, though other gases can be used. The extra argon ions created as a result of these collisions leads to a relatively higher deposition rate. It is known to arrange strong permanent magnets beyond the target in order to create such a magnetic field loop. At the location of the plasma loop on the surface of the target, a racetrack can be formed, which is the area of preferred erosion of material. In order to increase material utilization, movable magnetic arrangements are being used, that allow for sweeping the plasma loop over relatively larger areas of the target.
Direct current (DC) magnetron sputtering is a well-known technique using crossed electric and magnetic fields. An enhancement of DC magnetron sputtering is pulsed DC. The technique uses a so-called “chopper,” where an inductor coil L and a switch are used to modify a DC power supply into a supply of unipolar or bipolar pulses, see FIG. 1. The inductor coil L is the chopper and can preferably include a tap located between the DC power supply and the magnetron cathode. The electronic switch S is periodically open and closed to create the pulses. In the on-time of the switch S, an effective shortcut between the tap of the coil L and the magnetron anode switches the negative cathode voltage off, preferably overshooting to positive voltages by the auto transforming effect of coil L. During the off-time, the current from the DC power supply continues to flow into the coil L and storing energy in its magnetic field. When the switch S is again off, a short negative high voltage peak is formed at the magnetron cathode. This helps for relatively fast reigniting of the magnetron plasma and restoring the original discharge current.
The High Power Impulse Magnetron Sputtering (HIPIMS) technology as described in the prior art uses relatively lower repetition frequency of pulses typically 5 Hz to 200 Hz, and pulse times 20 to 500 μs. The discharge peak currents range from 100 A for relatively small cathodes up to 4 kA for relatively large cathodes, which corresponds to current density at cathode in the order of magnitude of 0.1 to 10 A/cm2. A common technique uses wiring as in FIG. 1. The work piece holder is either on an external potential, such as a DC potential, or the work piece holder is left on a floating potential in the plasma. The prior art design of FIG. 1 involves applying a DC bias to the workpiece holder.
There are a number of disadvantages with the standard HIPIMS technique. While using DC bias helps to define ion energy at a substrate, a disadvantage occurs in that arcing may occur on the substrate. Arcing on the substrate causes wafer damage when wafer processing is used. Another disadvantage is that DC bias also does not work on electrically insulating surfaces, such as trenches and vias using oxide materials.
FIG. 2 shows the result of an experiment. The data shows the measured rise time of the current as a function of frequency in a state of the art HIPIMS discharge. The target in this example was made of Tantalum (Ta), with the target having a diameter of 300 mm, and the experiment was using a rotating magnet array. For relatively low repetition frequency of 10 Hz (100 ms period), there is a relatively long delay (about 5 μs) between the start of the voltage pulse and the start of the current rise. The delay is somewhat shorter (over 4 μs) when a repetition frequency of 100 Hz (10 ms period) is used. With a relatively higher frequency of 500 Hz (2 ms period), the current starts to rise much faster, within only about 1.5 μs.
It has been shown that a pre-ionization using a radio frequency (RF) inductively coupled plasma (ICP) by the use of a coil placed between the target and the substrate helps to shorten the rise times of the current in an HIPIMS application. Nevertheless, placing a bulky ICP coil between the magnetron and the substrate complicates the design, increases the probability of particle formation, and reduces the deposition rate due to the increased target-substrate spacing.